1. Field of the Invention
The present invention relates to an optical storage apparatus for positioning an objective lens for irradiating a medium with a light beam from a light source to a target position on a medium by a tracking control and a focusing control and performing reproduction, recording, or erasure. More particularly, the invention relates to an optical storage apparatus for measuring and correcting an offset in a servo error signal caused by a change in an amount of reflection light from a medium.
2. Description of the Related Arts
Attention is paid to an optical disk as a removable storage medium as a core of multimedia which has rapidly been developing in recent years, and a magnetooptic disk (MO), a phase change optical disk (PD), and the like exist. An optical disk drive using such an optical disk as a storage medium has an objective lens for irradiating a medium with a light beam from a laser light source, which is mounted on an actuator movable in the radial direction of the medium, and performs a servo control of positioning the light beam to a target track position on the medium and positioning the objective lens so as to achieve focus on the medium by moving the objective lens in an optical axis direction. Such a servo control is performed by generating a servo error signal (a tracking error signal and a focusing error signal) indicative of a deviation from the target position of the objective lens on the basis of reflection light from the medium and positioning the objective lens to the target position on the basis of the servo error signal (tracking control and focusing control). Since the servo error signal used for the servo control is generated by receiving the reflection light from the medium by a photodetector, an amount of light received by the photodetector changes according to an amount of the reflection light from the medium and a light emitting power itself of the laser light source. Consequently, an offset component is generated in the servo error signal, and the servo stability deteriorates. The phenomenon of occurrence of an offset in the servo error signal due to a change in the amount of reflection light received by the photodetector will be described as follows. Although the phenomenon will be described with respect to a track system here, the phenomenon with respect to the focusing system is similar.
FIG. 1 shows a construction of a tracking error signal generating circuit. Specifically, the tracking error signal generating circuit includes: a photodetecting unit 300 having a pair of photodetectors 302 and 304 for receiving reflection light from a medium and performing a photoelectric conversion; current-to-voltage converters 306 and 308 for converting currents ia and ib from the photodetectors 302 and 304 to voltages Va and Vb, respectively; a subtractor 310 for obtaining a difference (Vaxe2x88x92Vb) of the two voltage signals Va and Vb derived by the conversion by the current-to-voltage converters 306 and 308; an adder 312 for obtaining a sum (Va+Vb); and a divider 314 for performing division between outputs of the subtractor 310 and the adder 312 and outputting the result as a tracking error signal E10. Although there is a case that the output of the subtractor 310 is used as a tracking error signal, generally, in order to suppress variations in amplitude according to the reflection light amount, the output of the subtractor 310 is divided by the total amount of reflection light calculated by the adder 312, thereby making the amplitude of the tracking error signal constant. Consequently, an ideal output of the tracking error signal E10 is obtained as follows.
E10=(Vaxe2x88x92Vb)/(Va+Vb)
When individual differences in the current-to-voltage converters 306 and 308 are considered and it is assumed that a small offset Vofs exists on the output Va side, the track error signal E10 is obtained as follows.
E10={(Va+Vofs)xe2x88x92Vb}/{(Va+Vofs)+Vb}
The offset Vofs is a small offset voltage which is always constant irrespective of currents supplied to the current-to-voltage converters 306 and 308. When the outputs Va and Vb of the current-to-voltage converters 306 and 308 have sufficiently large values, an influence of the offset Vofs is small and an influence on the tracking error signal E10 is also a little. When the difference between each of the outputs Va and Vb and the offset Vofs is too small to ignore the relation between the outputs Va and Vb and the offset Vofs, a change amount of the offset with respect to the amplitude of the tracking error signal E10 becomes too large. Conventionally, to deal with the offset change in the tracking error signal due to variations in the amount of the reflection light from the medium, an offset correction is performed in such a manner that a tracking error signal is fetched by an A/D converter in a DSP (Digital Signal Processor), when a change in the reflection light amount is detected, an offset amount is measured, a correction amount is calculated from the offset amount and, after that, the correction amount is added to the tracking error signal in the DSP (JP11328696a and U.S. patent application Ser. No. 09/196,098).
FIG. 2 shows a conventional tracking servo control unit. The tracking error signal E10 generated in FIG. 1 is supplied to an adding circuit 200 for correcting an offset, and an arbitrary correction amount from a DSP 205 is added to the tracking error signal E10 to thereby correct the offset. Unnecessary frequency band components in an offset-corrected tracking error signal E11 are eliminated by a notching circuit 202 and a low pass filter 204 and, after that, the resultant signal is fetched as a tracking error signal E12 by an A/D converter 206 in the DSP 205. The DSP 205 supplies the tracking error signal fetched by the A/D converter 206 to a correction amount detecting unit 224. An offset amount caused by a change in the amount of the reflection light in a sector ID area at the time of reproduction or in a data area at the time of recording or reproduction is measured, a correction amount to cancel out the offset amount is calculated and added to the output of the A/D converter 206 at an addition point 222, and an offset-corrected tracking error signal E13 is outputted. The correction amount detecting unit 224 uses an MOXID signal E14 and a write gate signal E15 supplied as signals for detecting a change in the reflection light to an edge port 232. As shown in FIG. 3A, the MOXID signal is a logical signal which becomes at the H level in the data area in a medium sector and becomes at the L level in an ID area between sectors. Since the amount of reflection light decreases in the ID area, by using the MOXID signal, an offset is measured in the ID area and corrected. FIG. 3B shows the tracking error signal E10 inputted to the adding circuit 200, and an offset occurs in the ID area where the MOXID signal E14 becomes at the L level. FIG. 3C shows sampling timings of the A/D converter 206, the tracking error signal E12 in FIG. 3D which has passed the notching filter 202 and the low pass filter 204 is sampled at timings of arrows to be converted to digital data, and the digital data is fetched. The correction amount detecting unit 224 measures an offset amount from a difference between sample values before and after the detection start timing in the ID area, calculates a correction amount having an amplitude of FIG. 3E from the measured offset amount, outputs the correction amount for a predetermined time, and adds the correction amount at the addition point 222, thereby obtaining the offset-corrected tracking error signal E13 as shown in FIG. 3F. The track error signal E13 in which the offset caused by a change in the reflection light amount has been corrected passes through an input gain multiplying unit 208, a PID computing unit 210, an output gain multiplying unit 212, and a D/A converter 214 and is outputted from the DSP 205. By the output, a driving current is passed from a power amplifier 215 as a driver to a tracking coil 216 to thereby position an objective lens 220 mounted on an actuator 218 to the track center of a target track. The DSP 205 is also provided with a D/A converter 230 of an offset eliminating unit 226. A correction amount to cancel out an offset brought about by a cause other than the change in the reflection light amount is fixedly added to the adding circuit 200, thereby correcting the offset. At the time of recording or erasing, by using the MOXID signal E14 and the write gate signal E15, an offset associated with an increase in the reflection light in the data area subsequent to the ID area is measured and corrected.
In such a conventional tracking servo control unit, however, since the tracking error signal E12 read by the DSP 205 via the A/D converter 206 is derived by passing the tracking error signal E10 generated by the tracking error signal generating circuit of FIG. 1 to the filters such as the notching filter 202 and the low pass filter 204, as obvious from a comparison between the signal waveform of FIG. 3B before passing through the filters and that of FIG. 3D after passing through the filters, a delay occurs also in an offset in the tracking error signal in the ID area and the waveform becomes dull. Conventionally, the tracking error signal E12 having such a delay and a dull waveform is fetched by the A/D converter 206 into the DSP 205 and is subjected to the offset correction. It is consequently difficult to, for example, determine the timing of measuring the offset amount from the MOXID signal E14. Particularly, in the case of the recording process, an offset (decrease in the reflection light amount) is caused also by the ID area just before a recording process, and an offset (increase in the reflection light amount) is caused also by the recording operation performed in the data area. There is a limitation in the offset correction, so that a problem that the offset cannot be sufficiently cancelled out occurs. In recent years, the rotational speed of an optical disk tends to be increasing, so that reliability of the calculation of the correction amount based on the measurement of the offset amount and promptness of the correction effect of cancelling out the offset are required. From this viewpoint as well, the correcting process using the tracking error signal which has passed the filters is limited.
According to the invention, there is provided an optical storage apparatus in which a servo control at the time of reproduction, recording, and erasing is stabilized and stability of a whole drive is improved by accurately detecting and correcting an offset amount in a servo error signal caused in association with a change in an amount of reflection light.
The invention is directed to an optical storage apparatus having: an actuator capable of positioning an objective lens for irradiating a medium with a light beam from a light source to a target position on the medium; a servo error signal generating circuit which generate a servo error signal indicative of a deviation from a target position of the objective lens on the basis of reflection light from the medium; an offset correcting circuit (adding circuit) for correcting an offset by adding an arbitrary correction amount to a servo error signal outputted from the tracking error signal generating circuit; a filter which eliminate an unnecessary frequency component from the servo error signal outputted from the offset correcting circuit; and a servo control unit which position the objective lens to a target position on the medium on the basis of the servo error signal outputted from the filter.
(Fundamental Construction)
According to the invention, the optical storage apparatus is characterized by including: an offset measuring unit which receive the servo error signal E1 which does not pass through the filter and measuring an offset amount in the servo error signal, caused by a change in an amount of reflection light; and a correction amount calculating unit which calculate a correction signal to cancel out the offset amount and outputting the correction signal to perform correction to the offset correcting circuit for an offset generating period. According to the invention as described above, the tracking error signal before being passed to the filter is received, an offset is measured, and the correction amount is calculated from the measured offset and used for correction. Consequently, a change in the offset due to a change in the amount of reflection light directly appears in the tracking error signal before being passed to the filter. As a result, the offset amount can be detected with high precision, and time for applying the correction amount is relatively easily determined. In addition, the offset-corrected servo error signal is passed through the filters and fetched by a servo control unit by a DSP. Consequently, there is also an advantage that, even if the timing of the offset correction is deviated more or less and an offset remains, the offset is filtered by the filter after that, so that an influence of a slight deviation in the correction timing can be eliminated. As a result, tracking in the ID area at the time of reproduction and tracking in the data area at the time of recording or erasing becomes stable, and the stability of the entire drive is improved. Since all of the changes from the conventional technique can be dealt in the DSP, the invention can be realized without increasing the cost by adding a new circuit part or the like.
The offset measuring unit receives a light amount change detection signal indicative of a change in an amount of reflection light from the medium and measures an offset amount from a difference between a servo error signal just before a detection start timing of the light amount change detection signal and a servo error signal just after the detection start timing. In this case, the correction amount calculating unit outputs a correction amount calculated on the basis of the offset amount to the offset correcting circuit for a period of time in which a light amount change is detected from the light amount change detection signal. The offset measuring unit may receive a light amount change detection signal indicative of a change in an amount of reflection light from the medium and measures an offset amount from a difference between a servo error signal just before a detection start timing of the light amount change detection signal and a servo error signal at a time point after elapse of predetermined time T1 since the detection start timing. Consequently, the servo error signal in which an offset appears can be measured with reliability in a state where the tracking is in the ID area, so that accuracy and reliability of measurement of an offset are increased. The correction amount calculating unit may output a correction amount calculated on the basis of the offset amount to the offset correcting circuit for predetermined time T1 since the detection start timing of the light amount change detection signal. The correction amount calculating unit outputs an auxiliary correction amount obtained by multiplying a correction amount of last time by a constant smaller than 1 to the offset correcting circuit for a period of time from the detection start timing of the light amount change detection signal until a correction amount based on the offset detection is calculated and outputted. Consequently, even when there is a time delay between the appearance of the offset in the servo error signal and the start of the correction, by correcting the offset by using the auxiliary correction amount obtained by multiplying the correction amount of last time by a coefficient ranging, for example, 0.5 to 0.75, the effect of cancelling out the offset can be further increased. The offset measuring unit calculates an offset amount on the basis of a plurality of past detection results. Therefore, an adverse influence on the offset correction in the case where a change which is not purely due to the reflection light amount, such as a medium defect, appears in the servo error signal can be reduced.
(Offset Correction in ID Area)
The invention is constructed as follows to correct an offset in the ID area necessary at the time of reproduction. First, the offset measuring unit receives a first logical signal (MOXID signal) indicating whether a light beam following a track in a medium is in a data area to which data can be recorded or in an ID area recorded between sectors, and measures an offset amount in the servo error signal on the basis of a detection timing of the ID area in the first logical signal. The correction amount calculating unit outputs a correction amount calculated based on the offset amount to the offset correcting circuit so as to perform correction for a period of detection of the ID area by the first logical signal. Specifically, the offset measuring unit measures an offset amount from a difference between a servo error signal just before a start timing of detecting the ID area by the first logical signal and a servo error signal just after the start timing. The correction amount calculating unit outputs a correction amount calculated on the basis of the offset amount to the offset correcting circuit for predetermined time T1 since the start timing of detection of the ID area by the first logical signal. The offset measuring unit measures an offset amount from a difference between a servo error signal just before the start timing of the ID area detection by the first logical signal and a servo error signal at a time point after elapse of predetermined time T2 since the start timing. At this time, the correction amount detecting unit outputs a correction amount calculated on the basis of the offset amount to the offset correcting circuit for predetermined time T1 since the start timing of detection of the ID area by the first logical signal. The correction amount calculating unit outputs an auxiliary correction amount obtained by multiplying a correction amount of last time by a constant to the offset correcting circuit for a period of time since the start timing of detection of the ID area by the first logical signal until the correction amount based on the offset detection is calculated and outputted. Further, the offset measuring unit calculates an offset amount on the basis of a plurality of past detection results.
(Offset Correction during Recording or Erasing)
The offset measuring unit receives a first logical signal (MOXID signal) indicating whether a light beam following a track in a medium is in a data area to which data can be recorded or an ID area recorded between sectors and a second logical signal (write gate signal) indicating whether the apparatus is recording data to the medium or erasing data in the medium, and measures an offset amount in the servo error signal being recorded or erased on the basis of a first logical signal and a second logical signal. In this case, the correction amount calculating unit outputs a correction amount calculated on the basis of the offset amount measured during the recording or erasing to perform correction to the offset correcting circuit. As described above, offsets caused by light amount changes which are different in the ID area and the data area subsequent to the ID area occur during recording or erasing. By using two kinds of logical signals corresponding to the different light amount changes, the offset can be accurately measured and corrected. Specifically, the offset measuring unit measures an offset amount from a difference between a servo error signal just before a start timing of detection of the ID area by the first logical signal and a servo error signal just after start of recording or erasing by the second logical signal. The correction amount calculating unit outputs a correction amount calculated on the basis of the offset amount during the recording or erasing by the second logical signal to the offset correcting circuit. The offset measuring unit detects an offset amount from a difference between a servo error signal just before the start timing of detection of the ID area by the first logical signal and a servo error signal after elapse of predetermined time T3 since the start of recording or erasing by the second logical signal. Consequently, the servo error signal can be measured at the timing when the offset appears due to a change in the reflection light amount by the start of recording or erasing. In this case, the correction amount calculating unit outputs a correction amount calculated on the basis of the offset amount during the recording or erasing by the second logical signal to the offset correcting circuit. The correction amount calculating unit outputs an auxiliary correction amount obtained by multiplying a correction amount of last time by a constant to the offset correcting circuit for a period of time since the start timing of recording or erasing by the second logical signal until the correction amount based on the offset detection is calculated and outputted. The correction amount calculating unit cancels out the offset also in the period until the correction amount is outputted. The offset measuring unit calculates an offset amount on the basis of a plurality of past detection results, thereby suppressing an adverse influence due to a medium defect or the like. In the case of continuously recording or erasing data to/from a plurality of sectors, the correction amount calculating unit continuously uses a correction amount calculated for the first sector for second and subsequent sectors. Further, the servo error signal generating circuit is a tracking error signal generating circuit which generate a tracking error signal indicative of a deviation from a target position in a medium track center of the objective lens on the basis of reflection light from the medium. The servo error signal generating circuit also includes a focusing error signal generating circuit which generate a focusing error signal indicative of a deviation from a focus position of the objective lens on the medium on the basis of reflection light from the medium.